Membrane filtration is widely used to separate high molecular weight materials or fine suspended solids from water or other liquids. Applications include removal of heavy metals from liquid waste of metal processing and finishing operations, recovery of precious metals in the jewelry and photographic film processing industries, and clean up of wastewater for recycling or sewer discharge.
The typical industrial laundry provides clothing, towels, rags and the like to heavy manufacturing, electroplating and automotive industries, battery manufacturers, service industries, newspapers and printers, automobile garages, schools, and a variety of other commercial and retain businesses. When returned to the laundry, these items are contaminated with a range of pollutants, such as oil and greases, heavy metals and solvents. The U.S. Environmental Protection Agency has analyzed laundries and characterized their wastes, finding a general consistency in the types of priority pollutants, but a wide variability in waste concentration from laundry to laundry. Typical pollutant(s) and concentration(s) found in laundry wastewater include TOC (1000 mg/l), BOD (1300 mg/l), COD (5000 mg/l), suspended solids (1000 mg/l), oil and grease (1100 mg/l), lead (4.5 mg/l), zinc (3.0 mg/l), copper (1.7 mg/l), chromium (0.88 mg/l), nickel (0.29 mg/l), chloroform (3.3 mg/l), benzene (2.5 mg/l) perchlorethylene (9.1 mg/l) and toluene (5.2 mg/l).
Conventional treatment systems used for the pretreatment of such wastes prior to discharge to publicly owned treatment works (POTW's) use chemical coagulation and flocculation in conjunction with dissolved air flotation to separate flocculant solids from the water phase, coupled with vacuum filtration to dewater the float solids. These treatment systems are primarily designed to reduce the levels of heavy metals and emulsified oils and greases. Dissolved solids, color materials, and taste and odor compounds generally are not taken into consideration since the system effluent is commonly discharged to a POTW and not reused.
Using typical chemical coagulation and dissolved air flotation systems, wastewater treatment operating costs range from $4.00 to $5.00 per 1000 gallons, with treatment chemicals comprising the largest component costs, e.g., $2.00 to $3.00 per 1000 gallons. Additionally, typical water and sewer costs in 1983 were $0.60 and $0.90, bringing the total water/wastewater operating expense of the laundry operator to approximately $5.00 to $6.00 per thousand gallons.
Due to anticipated yearly increases in water and sewarage rate, and the practice on the part of POTW's to include industrial laundries in point-source programs, it would be highly desirable to provide a system capable of producing product water will be acceptable for discharge into POTW's and, preferably, which can be reused. Such a system should provide for the cost effective removal of colloidal and suspended solids, BOD (Biological Oxygen Demand), COD (Chemical Oxygen Demand), TOC (Total Organic Carbon), color, and odor producing compounds from, reduction of TDS (Total Dissolved Solids) in the wastewater. If the water is to be reused, the system should also result in an effluent containing only a relatively low level of dissolved calcium.
Table 1, below, sets forth typical water quality criteria for POTW discharge and for reuse in an industrial laundry.
TABLE 1 ______________________________________ TYPICAL POTW DISCHARGE RECYCLE ITEM CRITERIA CRITERIA ______________________________________ BOD 100 mg/l 30 mg/l COD usually not limited 100 mg/l OIL & GREASE 100 mg/l 10 mg/l SUSPENDED 250 mg/l none allowable SOLIDS LEAD 1 mg/l 0.1 mg/l ZINC 1 mg/l 0.1 mg/l COPPER 1 mg/l 0.1 mg/l CHROMIUM 1 mg/l 0.1 mg/l NICKEL 1 mg/l 0.1 mg/l CHLOROFORM 1 mg/l 0.1 mg/1 BENZENE 1 mg/l 0.1 mg/l PERCHLOR- 1 mg/l 0.1 mg/l ETHYLENE TOLUENE 1 mg/l 0.1 mg/l pH 6-9 7-8 HARDNESS Usually not regulated 50 mg/l as CaCO3 IDS Usually not regulated 2000 mg/l COLOR Usually not regulated None allowable ODOR Usually not regulated None allowable ______________________________________